It is believed by many that OES (Original Equipment Supplier) parts have to be purchased directly from the dealer. OES parts are made on the same assembly line as the OEM(Original Equipment Manufacturer) except it goes through a couple added steps. Branding of the part or stamping with the manufacturer logo IE: HONDA, and then gets wrapped and labeled in the manufacturers packaging. This OES part has just become more expensive to the consumer because of theses added steps and inflated by numerous middlemen. Due to the regulations of the ISO (International Organization for Standardization) certification there are quite a few OEM companies on the market because multiple OES brands are required. For the Auto manufacturers to have a steady supply on the production line, they must have several supply sources that provide them with equal quality parts. This ensures if a manufacturer has any problems with one supplier it will not disable the vehicles production and allow for substitution of other brands for production, repairs and warranty replacement.
Aftermarket parts are copies of the OEM part. These parts are made to be a more economically feasible replacement parts than an OEM part. These parts are usually carried by your retail “chain” parts suppliers and are intended to be sold to the “do it yourselfer” who is more concerned with price than quality or longevity. However in my experiences there are some aftermarket companies that take OEM parts to the next level of quality. MOOG suspension components being a good example, they take for instance a ball joint, reverse engineer the part and and then recreate a replacement part that is of a higher quality. These products are most of the time equivalent in price to an OES part but carry an outstanding lifetime warranty.
In conclusion, there are several options to meet your repair, service and budget needs without sacrificing overall quality.